The effect of oxygen addition to chlorine plasma during shallow trench isolation etching is quantified in this work. Specifically, the electron density and the electron temperature in an electron cyclotron resonance reactor were characterized by a Langmuir probe and were found to remain relatively constant upon O2 addition. The silicon etching rates were found to increase with the square root of the ion energy, suggesting the etching reaction is limited by the momentum transfer from ions to the surface. A relatively small amount of oxygen addition (<10%) to the chlorine plasma simultaneously changes the reactor wall conditions and surface kinetics, since oxygen becomes actively involved in the surface reactions. The change in the chamber wall conditions and surface kinetics leads to the change in both the amount of etch products and the etched feature profile. The incorporation of oxygen on the surface results in a significant change of the etched surface morphology and its composition. This work suggests a small amount of O2 addition to Cl2 plasmas in shallow trench isolation etching changes the etching behavior primarily through modifying the kinetics on etched surfaces. A multiscale etch model consisting of translating mixed layer and Monte Carlo modules for bulk and feature scale etching, respectively, was successfully applied to this case, demonstrating good agreement with the experimental results.